1. Field of the Invention
The present invention relates to an optical modulator used in a transmission apparatus for transmitting high speed digital signals in an optical communication system using optical fibers as a transmission channel.
In recent years, optical communication systems using optical signals as the medium of communication have rapidly been put into use. The optical modulators for producing the optical signals in the optical communication systems are extremely important devices. The present invention refers to such optical modulators, in particular external modulation type optical modulators.
2. Description of the Related Art
In the conventional direct modulation type optical modulators, when the modulation speed became large, so-called frequency chirping occurred, resulting in deterioration of the high speed characteristics. Frequency chirping is the shift in the wavelength of the light during the rise and fall of the light signal with each such rise and fall. Frequency chirping causes waveform deterioration in the light signal received in the receiving system through the optical fiber due to optical fiber wavelength dispersion. Therefore, in a high speed optical communication system wherein a high speed light modulation of several Gbps is required, it is not possible to use a direct modulation type optical modulator.
One known type of external modulation type optical modulator is a Mach-Zehnder interferometer type modulator, which will be explained later. Such a Mach-Zehnder interferometer type modulator has the smallest spread of the spectrum and therefore is able to avoid the waveform deterioration at the receiver caused by the effects of wavelength dispersion of the fiber. That is, in such a Mach-Zehnder interferometer type modulator, the phases of the light propagating through two optical waveguides are modulated in opposite directions by the same magnitude for modulation free from frequency chirping and it is thus possible to reduce the spectral spread to the spread of the modulation side band, by the Fourier component of the modulation waveform. Note that a known reference relating to an optical modulator of this type is F. Koyama et. al., JOURNAL OF LIGHTWAVE TECHNOLOGY, vol. 6, No. 1, Jan. 1988, IEEE, pp. 87 to 93.
There are, however, the following problems in the conventional Mach-Zehnder interferometer type modulator.
First, while there is a desire to make the modulation driving circuit of the optical modulator by a semiconductor integrated circuit (IC) and thus reduce the size, it is not easy to make the modulation driving circuit by an IC chip. The reason is that the conventional Mach-Zehnder interferometer type modulator requires a high voltage of -5 V to -8 V and it is difficult to manufacture an IC able to withstand such a high voltage.
Second, while there is a desire to reduce the offset of the optical modulator as much as possible, it is not easy to reduce the offset. Here, "offset" means the offset in the modulation driving voltage and deviation in various characteristics accompanying temperature fluctuations.
Third, it is not possible to use an optical modulator for a long distance optical communication system operating at a high transmission speed of over several Gbps. This is due to the so-called wavelength dispersion in the optical fibers. That is, it has not been possible to meet the demand for constructing a high speed, long distance optical communication system using such general optical fibers producing large wavelength dispersions.